1. Field of the Invention
The present invention relates to a voltage & displacement measuring apparatus and its probe for measuring the position and voltage of a micro structure such as a wiring on a semiconductor chip.
2. Description of the Related Art
In order to measure voltages in the fine wiring on semiconductor integrated circuits, there are some measuring instruments using an electron beam, namely, an electron beam tester, and using a light beam, namely, a light beam tester.
An electron beam tester emits an electron beam on to a fine wire to be measured (measuring point) and detects the quantity and energy of secondary electrons emitted from this measuring point. The voltage is measured based upon this detection.
A light beam tester uses the electro-optic effect of crystals; by radiating a light beam into an electro-optic crystal which is positioned near the measuring point and detects the change of the polarization status of the light passed through the crystal, and the voltage is measured based upon this detection.
For measuring the waveform of a voltage pulse with a short leading edge or trailing edge time such as 1 nano-second or less, with an electronic or light beam tester, a pulsed beam is used to sample the voltage at the time when the beam is irradiated.
With an electron beam tester, on the one hand, it is necessary to raise the spatial resolution by narrowly constricting the electron beam in comparison with the width of the wire at the measuring point while, on the other hand, it is also necessary to raise the time resolution by setting a short pulse width for the electron beam, in order to respond to rapid changes in the waveform being measured. Because of this, the number of electrons in the pulse beam is reduced and this, in turn, reduces the number of secondary electrons generated at the beam irradiation points, resulting in a deterioration of the S/N ratio (the voltage resolution is reduced). To deal with this, a cyclical test signal may be supplied to the semiconductor integrated circuit which is being measured, and a pulse beam is then radiated in the same phase for several cycles, and the average value of the detection signal can be used to improve the S/N ratio. However, this measuring procedure requires a long time.
Since the S/N ratio is reduced when the pulse width of an electron beam is narrower, and since the secondary electron detection signal spreads in the direction of time because there are differences in running time among secondary electrons, the upper limit of time resolution at present is approximately 5 pico-seconds and the realization of higher time resolution is difficult to achieve.
The light beam tester provides a higher time resolution, such as 0.5 pico-seconds, and a higher voltage resolution, namely higher S/N ratio, over the same values obtained for electron beam tester. However, since the spatial resolution is limited by the wavelength of the light, it is difficult to measure the voltage in a fine wire. Actually, it is possible to measure the voltage in wire having a diameter of 1 .mu.m or more, at present, but it is difficult to measure voltage in any wire having a diameter less than that.
As has been explained, the electron beam tester provides good spatial resolution but is unsatisfactory in time while, in contrast, resolution and measuring time, the light beam tester excels in time resolution and measuring time, but is unsatisfactory in spatial resolution. Namely, these two types of testers have opposite and complementary advantages and shortcomings.
Furthermore, when probing a narrow wire on the order of sub-micron dimensions, with a light beam tester, the electrical contact with the wire is insufficient.
A probe for a voltage and displacement measuring apparatus which can improve the space and time resolution, would be conceived, the schematic structure of which is shown in FIG. 19.
The semiconductor chip 10 has the bonding wires 11, 12 connecting the pad on its surface and the inner lead (not shown).
The upper end of the cantilever 22 of the probe 20, is bonded to the lower end of the substrate 21 and the electrically conductive probing needle 23 is bonded to the lower end of the cantilever 22. The cantilever 22 and the probing needle 23 are of the type that are used in AFM (Atomic Force Microscope) and the cantilever 22 has the spring constant of 1 to 100 N/m and is, therefore, soft enough to measure. The diameter of the probing needle 23 at the top is very small (e.g., 50 nanometers or less). When the probing needle 23 is placed close to the semiconductor chip 10, a distance in the order of nanometers, or when the probing needle 23 contacts the semiconductor chip 10, displacement occurs in the cantilever 22. Displacement in the direction of the height of the probing needle 23 is measured by irradiating a laser beam from the laser 24 on to the upper surface of the lower end of the cantilever 22 and by detecting its reflected light with the PSD (Position Sensitive Detector) 25. While measuring this displacement, an X-Y scanning of the probe 20 relative to the semiconductor chip 10 is performed by applying a piezoelectric actuator. During the X-Y scan, the displacement of the probing needle 23 can be constant by controlling the Z-direction piezoelectric actuator, and an image of the concave-convex surface of the chip 10 can be obtained based upon the driving signal for the Z-direction piezoelectric actuator. This image will have a spatial resolution in the order of nanometers and the position of the probing needle 23 on the wire to be measured can be determined with an accuracy in the order of nanometers based upon this image.
In order to detect the potential at the measuring point, the electro-optic crystal 26 is bonded onto the substrate 21. The lower surface of the electro-optic crystal 26 is electrically connected with the probing needle 23 by an electrically conductive film and the electrically conductive transparent film which is adhered to the upper surface of the electro-optic crystal 26 is grounded. When the probing needle 23 contacts the measuring point on the wire or approaches the measuring point to within a distance on the order of nanometers, an electric field, due to the potential of the wire, is applied to the electro-optic crystal 26. The laser beam from the laser 28, which has been reflected by the mirror 27, enters the electro-optic crystal 26 and then the light reflected from the bottom of the electro-optic crystal 26 via a polarization beam splitter (not shown) is detected by the photodetector (not shown). The potential at the measuring point is measured based upon this detection.
However, since the tip of the probing needle 23 must either contact the semiconductor chip 10 or be in and after close proximity to it, there is a limit to the angle of inclination of the substrate 21 relative to the semiconductor chip 10. In addition, the probing needle 23 must scan the semiconductor chip 10 in such a manner that the substrate 21 does not interfere with the bonding wires 11 and 12. Accordingly, the probing needle 23 cannot scan in the range R shown in FIG. 19, resulting in an area that cannot be measured.
Also, since a mechanism is required that rotates the probe 20 or the chip 10 in correspondence with the position of the probe 20 on the chip 10 so that the probe does not interfere with the bonding wires, the structure and operation of the apparatus become complicated.
Furthermore, since an optical system for displacement measurement and a separate optical system for voltage measurement are required, the structure and the adjustment thereof become complicated.